add supporing handlers with res parameter

void handler(const crow::request&, crow::response& res)
{
	res.send(data)
	res.end()
}

https://news.ycombinator.com/item?id=8004017
long polling implementation complete

change `res handle(req)' into `void handle(req, res)'
connnection::handle is divide into two part:
    before and after user handler